This invention relates to negative air filtration systems, and more particularly to an adaptor and control to provide a negative air pressure in a glove bag containment unit for removal of hazardous materials.
For many years it has been customary whenever necessary to work on or remove hazardous materials from an area such as a room or building to form an enclosure around the work area and to maintain a negative air pressure in said enclosure relative to the ambient. Under these conditions, if there should be a breech in the enclosure, the hazardous material will not escape, but rather additional air will be drawn into the chamber and exhausted through conventional protective filtering mechanism. For many years small removal jobs such as small amounts of asbestos on pipes and the like have been accomplished by sealing a plastic enclosure around the area to be worked on with suitable integral gloves through which hands can be inserted to work on the hazardous materials without allowing the escape of any of the hazardous material.
Recently, it has been required by OSHA, and others interested in the best possible practice in the removal of and work on hazardous materials, that even "glove bags" used in small jobs must have a negative air pressure maintained within the enclosure to further insure against escape of any contaminated material. Since glove bags are normally made of a thin plastic transparent material, and generally enclose only a very small volume area, the provision of a negative pressure by applying a vacuum to the enclosure, very quickly collapses the air bag so that no further work can be accomplished within the bag.
Accordingly, in an effort to continue to use the economical glove bag approach while allowing sufficient time to perform the necessary operations within the glove bag, it has been proposed to draw down a glove bag to the negative pressure very slowly so as to maintain a negative pressure differential from outside to inside, but to limit the rate at which this negative pressure is built up so that 15 to 30 minutes of operating time can be obtained within the glove bag before it collapses about the work.
In most asbestos removal applications there are present on-site vacuum blowers with suitable HEPA filters to trap and retain any hazardous fibers in the exhausted air. It has been proposed to use them to also provide the negative pressure within a glove bag unit. Because of the larger size of these units and the fact that to operate efficiently they must be run at a relatively high flow volume, it has been proposed that the connection to a glove bag be greatly restricted, and that the balance of the makeup air necessary for the efficient running of the vacuum machine be drawn through the surrounding environment while only a small proportion of air is drawn from the interior of the glove bag. One device for accomplishing this has been a small metal tube adaptor for a HEPA vacuum hose which consisted of an outer tube approximately six inches long and one-and-one-half inches in diameter and an inner tube about two inches long and three-eighths of an inch in diameter that fits within the larger one and which is inserted into the glove bag to be evacuated. This smaller tube is restricted in opening so that the air drawn from the glove bag is drawn relatively slowly and the balance of the air necessary for the running of the vacuum is drawn through the larger tube surrounding the smaller tube so that the vacuum machine can still be run efficiently and yet the glove bag will not be immediately collapsed. Devices of this type have provided up to one-half hour of operating time for removal or other remediation of hazardous materials within the glove bag while insuring against unwanted escape of hazardous fibers.
These devices have proven very helpful but they have a potentially hazardous characteristic in that if the evacuation system should be stopped for any reason, after the negative air pressure is equalized by an inrush of air through the larger pipe to the smaller one, the further application of external pressure on the glove bag could force contaminated or hazardous particles within the glove bag back out in reverse fashion through the large pipe which is open to the ambient atmosphere.